(1) Field of the Invention
This invention relates to the catalytic hydrogenation of tall oil rosin. More particularly, this invention involves the partial hydrogenation and sulfur reduction of tall oil rosin in the presence of a palladium catalyst.
(2) Description of the Prior Art
The catalytic hydrogenation of rosin using both base metal catalysts and noble metal catalysts is known. The rosins for which economical commercial processes have been developed are of either the wood or gum variety. Wood rosins are obtained by solvent extraction from the stumps of the Jeffrey and Ponderosa pines, distilling off the volatile fraction. Gum rosin is the residue obtained after the distillation of turpentine oil from crude turpentine oleoresin.
Tall oil rosin, to which this invention relates, is a product in the fractionation of tall oil which is extracted from the black liquor resulting from alkaline pulping processes. While all rosins are composed of resin acids of the abietic and pimaric types and an unsaponifiable portion, these constituents vary greatly according to the source of rosin, i.e., wood, gum or tall oil.
Because of its complex mixture of several isomers and poorly characterized non-rosin impurities (unsaponifiable portion may be up to 15% of the total), tall oil rosin is particularly difficult to hydrogenate. The considerable impurities contribute to poor color characteristics and act to poison the hydrogenation catalyst. During hydrogenation, the rosin may undergo decarboxylation and isomerization and these reactions are competitive to hydrogenation. The decarboxylation leads to a loss of acid number and softer products that fume when heated. Isomerization leads to rosin isomers that are resistant to hydrogenation.
Tall oil rosin's resistance to hydrogenation is taught in U.S. Pat. No. 2,776,276 which recommends pre-treating tall oil rosin prior to hydrogenating to remove catalyst poisoning constituents. No pre-treatment is suggested for wood or gum rosins. The patent teaches hydrogenation with a 5% palladium-on-activated charcoal catalyst at a temperature of from 125.degree. C. to 300.degree. C. and a pressure of at least 3000 psig to produce a substantially completely hydrogenated product, which is defined as a hydrogenated rosin which has absorbed at least 1.22% hydrogen based on the unhydrogenated rosin. A palladium to rosin ratio as small as 1:2000 on a parts by weight basis is taught for anhydrous conditions.
The patentees also published "Catalytic Perhydrogenation of Rosin" in Industrial and Engineering Chemistry, Vol. 50, No. 3, 313-316, March 1958. Perhydrogenation is defined as meaning the hydrogenation of all double bonds and aromatic rings, leaving only tetrahydroabietic type acids and no dehydroabietic acids. The perhydrogenation reaction conditions employed are identical to those recommended in the patent, and, therefore, "substantially complete hydrogenation" in the patent refers to hydrogenation of dehydroabietic acid also. This is consistent with the 1.22% minimum hydrogen absorption specified in the patent.
It is an object of this invention to provide a process for partially hydrogenating tall oil rosin in a single hydrogenation step without tall oil rosin pre-treatment. The process may be operated in either a batch or continuous mode. The partial hydrogenation of tall oil rosin is achieved by the selective reduction of only olefinic double bonds; thus, the invention process does not involve saturation of the dehydroabietic acid aromatic double bonds.
A further potential hindrance to hydrogenation which is somewhat unique to tall oil rosin is its considerable sulfur content, attributable to its derivation from kraft pulping liquors. Sulfur compounds are taught as catalyst poisons in the above discussed paper on perhydrogenation, in U.S. Pat. No. 3,655,635 with respect to tall oil rosin, and in U.S. Pat. No. 2,727,885, with respect to unfractionated tall oil. Inasmuch as sulfur may be an undesirable end product constituent for certain applications, its removal may be beneficial for reasons other than catalyst affinity, as discussed in U.S. Pat. No. 3,489,740.
The economics of any catalytic process may be drastically affected by catalyst consumption. This is especially true in the case of an expensive noble metal catalyst, such as palladium. In fact, in "Catalytic Perhydrogenation of Rosin," which employs a palladium-on-carbon catalyst and wood rosin, the authors concluded that to obtain an economical catalyst life the rosin must be purified before use.
Therefore, it is a further object of this invention to provide an economical single step process for the simultaneous partial hydrogenation and sulfur reduction of tall oil rosin without pre-treatment. Further still, since an end product acid number lower than that of the starting material is indicative of decarboxylation, as opposed to hydrogenation, it is an object of this invention process to provide a partially hydrogenated tall oil rosin with minimal acid number reduction.